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Employees, Retirement and Incentive Plans Performance Units (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Share-based Compensation Arrangement Instruments [Roll Forward]    
Weighted Average Fair Value of Units Outstanding $ 1.2  
Performance Shares [Member]    
Share-based Compensation Arrangements    
Estimated Share Payouts Unit Awards Minimum 100.00%  
Estimated Share Payouts Unit Awards Maximum 150.00%  
Share-based Compensation Arrangement Instruments [Roll Forward]    
Outstanding at January 1, 2020 (nonvested) 53,445  
Vesting and transfer of common units to recipients (11,634)  
Outstanding at March 31, 2020 (nonvested) 41,811  
Fair value of vested units transferred to recipients $ 0.4 $ 0.3
Total unrecognized compensation related to nonvested units $ 0.4  
Weighted average remaining contractual term (years) 1 year 7 months 6 days  
Restricted and Phantom Units    
Share-based Compensation Arrangement Instruments [Roll Forward]    
Outstanding at January 1, 2020 (nonvested) 145,205  
Vesting and transfer of common units to recipients (4,397)  
Forfeitures (5,890)  
Outstanding at March 31, 2020 (nonvested) 134,918  
Total unrecognized compensation related to nonvested units $ 1.4  
Weighted average remaining contractual term (years) 1 year 2 months 12 days